53 research outputs found
Correlations among global photometric properties of disk galaxies
Using a two-dimensional galaxy image decomposition technique, we extract
global bulge and disk parameters for a complete sample of early type disk
galaxies in the near infrared K band. We find significant correlation of the
bulge parameter n with the central bulge surface brightness and with
effective radius r_e. Using bivar iate analysis techniques, we find that , and are distributed in a plane with small scatter. We
do not find a strong correlation of n with bulge-to-disk luminosity ratio,
contrary to earlier reports. r_e and the disk scale length r_d are well
correlated for these early type disk galaxies, but with large scatter. We
examine the implications of our results to various bulge formation scenarios in
disk galaxies.Comment: 14 pages, LaTeX including 14 figures. To appear in the Astrophysical
Journa
Einstein Cluster Alignments Revisited
We have examined whether the major axes of rich galaxy clusters tend to point
toward their nearest neighboring cluster. We have used the data of Ulmer,
McMillan, and Kowalski, who used position angles based on X-ray morphology. We
also studied a subset of this sample with updated positions and distances from
the MX Northern Abell Cluster Survey (for rich clusters () with well
known redshifts). A Kolmogorov-Smirnov (KS) test showed no significant signal
for nonrandom angles on any scale Mpc. However, refining the
null hypothesis with the Wilcoxon rank-sum test, we found a high confidence
signal for alignment. Confidence levels increase to a high of 99.997% as only
near neighbors which are very close are considered. We conclude there is a
strong alignment signal in the data, consistent with gravitational instability
acting on Gaussian perturbations.Comment: Minor revisions. To be published in Ap
Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: II. Rotating Versus Non-Rotating Dwarfs
We present spatially-resolved internal kinematics and stellar chemical
abundances for a sample of dwarf elliptical (dE) galaxies in the Virgo Cluster
observed with Keck/ESI. We find that 4 out of 17 dEs have major axis rotation
velocities consistent with rotational flattening, while the remaining dEs have
no detectable major axis rotation. Despite this difference in internal
kinematics, rotating and non-rotating dEs are remarkably similar in terms of
their position in the Fundamental Plane, morphological structure, stellar
populations, and local environment. We present evidence for faint underlying
disks and/or weak substructure in a fraction of both rotating and non-rotating
dEs, but a comparable number of counter-examples exist for both types which
show no evidence of such structure. Absorption-line strengths were determined
based on the Lick/IDS system (Hbeta, Mgb, Fe5270, Fe5335) for the central
region of each galaxy. We find no difference in the line-strength indices, and
hence stellar populations, between rotating and non-rotating dE galaxies. The
best-fitting mean age and metallicity for our 17 dE sample are 5 Gyr and Fe/H =
-0.3 dex, respectively, with rms spreads of 3 Gyr and 0.1 dex. The majority of
dEs are consistent with solar alpha/Fe abundance ratios. By contrast, the
stellar populations of classical elliptical galaxies are, on average, older,
more metal rich, and alpha-enhanced relative to our dE sample. The local
environments of both dEs types appear to be diverse in terms of their proximity
to larger galaxies in real or velocity space within the Virgo Cluster. Thus,
rotating and non-rotating dEs are remarkably similar in terms of their
structure, stellar content, and local environments, presenting a significant
challenge to theoretical models of their formation. (abridged)Comment: 33 pages, 12 figures. To appear in the October 2003 Astronomical
Journal. See http://www.ucolick.org/~mgeha/geha_dE.ps.gz for version with
high resolution figure
A re-assessment of the nearest neighbour alignment of the X-ray isophotes of galaxy clusters
Alignment is defined as the tendency of the distribution of pointing angles between the major
axes of clusters and their nearest neighbours to be more concentrated towards small values
for small nearest neighbour distances, whereas the distribution is expected to be uniform over
all angles at larger distances. Conflicting pronouncements on the reality of this effect have
been published in the astronomy literature. A re-assessment of the evidence for alignment is
presented, based on three recently published X-ray data sets. We find that whereas there is
evidence for alignment, it is not as convincing as previously claimed. In particular, the scale
to which the effect has been claimed to extend seems to have been severely overstated
N-body simulations of galaxies and groups of galaxies with the Marseille GRAPE systems
I review the Marseille GRAPE systems and the N-body simulations done with
them. I first describe briefly the available hardware and software, their
possibilities and their limitations. I then describe work done on interacting
galaxies and groups of galaxies. This includes simulations of the formation of
ring galaxies, simulations of bar destruction by massive compact satellites, of
merging in compact groups and of the formation of brightest members in clusters
of galaxies.Comment: 13 pages, 5 figures, to be published in "Non-linear Dynamics and
Chaos in Astrophysics", eds. J.R. Buchler, S. Gottesman, J. Hunter and H.
Kandrup, Annals of the New York Academy of Science
Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: I. A Keck/HST Study of Six Virgo Cluster Dwarfs
Keck/ESI spectroscopy is presented for six Virgo Cluster dwarf elliptical
(dE) galaxies. The mean line-of-sight velocity and velocity dispersion are
resolved as a function of radius along the major axis of each galaxy, nearly
doubling the total number of dEs with spatially-resolved stellar kinematics.
None of the observed objects shows evidence of strong rotation: upper limits on
v_rot/sigma are well below those expected for rotationally-flattened objects.
Such limits place strong constraints on dE galaxy formation models. Although
these galaxies continue the trend of low rotation velocities observed in Local
Group dEs, they are in contrast to recent observations of large rotation
velocities in slightly brighter cluster dEs. Using WFPC2 surface photometry and
spherically-symmetric dynamical models, we determine global mass-to-light
ratios 3 < M/L_V < 6. These ratios are comparable to those expected for an
intermediate-age stellar population and are broadly consistent with the (V-I)
colors of the galaxies. This implies that these dEs do not have a significant
dark matter component inside an effective radius. Central black holes more
massive than 10^7 M_sun can be ruled out. For the 5 nucleated dEs in our
sample, we determine kinematic and photometric properties for the central
nucleus separately from the underlying host dE galaxy. These nuclei are as
bright or brighter than the most luminous Galactic globular clusters and lie
near the region of Fundamental Plane space occupied by globular clusters. In
this space, the Virgo dE galaxies lie in the same general region as Local Group
and other nearby dEs, although non-rotating dEs appear to have a slightly
higher mean mass and mass-to-light ratio than their rotating counterparts.Comment: 31 pages, 11 figures. Accepted in the Astronomical Journa
A combined XAS and XRD Study of the High-Pressure Behaviour of GaAsO4 Berlinite
Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD)
experiments have been carried out on GaAsO4 (berlinite structure) at high
pressure and room temperature. XAS measurements indicate four-fold to six-fold
coordination changes for both cations. The two local coordination
transformations occur at different rates but appear to be coupled. A reversible
transition to a high pressure crystalline form occurs around 8 GPa. At a
pressure of about 12 GPa, the system mainly consists of octahedral gallium
atoms and a mixture of arsenic in four-fold and six-fold coordinations. A
second transition to a highly disordered material with both cations in six-fold
coordination occurs at higher pressures and is irreversible.Comment: 8 pages, 5 figures, LaTeX2
Stars, Star Clusters, and Dust in NGC 3077
Images obtained with the CFHTIR camera on the Canada-France-Hawaii Telescope
are used to investigate the near-infrared photometric properties of the
star-forming M81 group galaxy NGC 3077. The spectral-energy distribution (SED)
of the near-infrared light within 10 arcsec of the nucleus is consistent with
the 2um light being dominated by hot young (log(t_{yr}) < 6.8) stars reddened
by A_V = 3 - 4, with A_V < 8 mag in some regions. A population like that near
the center of NGC 205 likely contributes only a modest fraction of the light
near 2um. A number of candidate star clusters are detected in and around NGC
3077. The specific frequency of globular clusters falls within the range
measured in nearby dEs. The candidate young clusters have photometric masses
that are similar to those of compact young clusters in other active
star-forming systems, and SEDs consistent with ages log(t_{yr}) < 6.6. Based on
the masses and ages of the young clusters, it is estimated that the star
formation rate in NGC 3077 was at least 0.25 - 0.50 solar masses per year
during the past few million years.Comment: 26 pages + 7 postscript figures; to appear in the Astronomical
Journa
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